The present invention relates to a method for preparation of polyolefin oils, and particularly relates to a method for preparation of polyolefin oils by employing a mixture of ether derived from polyhydric alcohol and aluminum halide as polymerization catalysts, which can perfectly prevent halogen from being present in the polymerization product and provide polyolefin oils of high viscosity index at a high yield.
The art of employing Lewis acid such as aluminum chloride as catalyst in effecting polymerization of olefins has been known for log. However, the conventional Lewis acid catalysts have been defective in that, inasmuch as they are soluble in olefins, it is not easy to separate the catalyst at the time of completing the reaction, and even if the catalyst is separated and removed by the use of an aqueous solution of caustic alkali, an aqueous ammonia or an adsorbent, it is quite difficult to reuse the thus recovered catalyst for the purpose of polymerizing olefins.
Japanese Pat. Publication No. 9657/1967 discloses a method of polymerizing olefins which comprises effecting polymerization of an olefin-type unsaturated compound capable of cationic polymerization by employing a catalyst consisting of non-volatile residue obtained through thermal cracking of a compound to be expressed by the general formula AlX.sub.3 (R.sub.2 0).sub.n [wherein X represents halogen, R represents aliphatic or aromatic hydrocarbon radical and n is an integer of 1 or more]. Hoever, in the case of polymerizing olefins having 6 or more carbon atoms by this method, not only the reaction temperature is required to be 150.degree. C or more, but also the yield is low. Besides, the resulting polyolefin oil is defective in that it contains much halogen mixed therein and the viscosity index thereof is low.
Japanese Pat. Publication No. 3804/1969 discloses a method of polymerizing olefins in the presence of a catalyst prepared by dissolving excess aluminum halide in a complex consisting of aluminum halide and ethyl acetate at the molar ratio of 1:1 . However, as the foregoing complex consisting of aluminum halide and ethyl acetate can act as no more than a solvent for dissolving aluminum halide, separation of the catalyst from the reaction product after completing the reaction is difficult like in the case of employing aluminum halide independently, and moreover it is necessary to add aluminum halide anew when said complex is assigned for reuse.
In the light of the drawbacks of the prior art as stated above, the present inventors have previously proposed a method of preparing polyolefin oils by employing a granular solid catalyst to be obtained by mixing an olefin in an amount of 1 mole or more and a ketone having 4 or more carbon atoms in an amount of 1.0 - 1.2 mole with 1 mole of an aluminum halide and making them react at a temperature of more than 60.degree. C (cf. U.S. Pat. No. 3,953,361). This method has advantages such that it facilitates the recovery of catalyst after the reaction and the recovered catalyst can be reused as it is, but it still leaves room for improvement because the yield is in the range of about 70 - 80% and is not very high.
The present inventors have thereafter proposed, in U.S. Pat. No. 3,952,071, a method of preparing polyolefin oils which comprises polymerizing olefins having 6 or more carbon atoms in the presence of a mixture of a polyhydric alcohol derivative and an aluminum halide, said derivative being obtained by replacing the hydrogen atoms of all the hydroxyl groups of the polyhydric alcohol either by acyl groups exclusively or by acyl groups and alkyl groups, both of said groups having 1 - 20 carbon atoms.
With a view to developing a further improved method of polymerizing olefins which will not only facilitate the recovery of the polymerization catalyst after the reaction and render it possible to reuse the recovered catalyst but also produce polyolefin oils at a high yield, the present inventors have conducted a series of studies subsequent thereto. As a result, they have come to the finding that the employment of a mixture of ether derived from polyhydric alcohol and aluminum halide as catalyst in polymerization renders it possible to prepare polyolefin oils having a high viscosity index and containing no halogen at all at a high yield or more than 90% and facilitates separation of the catalyst from the reaction product, and moreover, the thus recovered catalyst, when reused as it is, renders it possible to maintain a high yield practically equivalent to that in the initial polymerization reaction.